In a resonant discontinuous power converter a magnetic energy storage device, in many cases a transformer, is used to cyclically transfer energy from an input to an output of the converter, broadly speaking by switching power through the device. The converter may be a forward converter, when power is transferred to the output when the switch is on, or a flyback converter when power is transferred whilst the switch is off. In either case when the switching device is on the magnetic flux in the device builds up (the flux is positive) but in a subsequent portion of the cycle, when the switch is off, the flux decays. Moreover the inductance of the transformer can resonate with a capacitance during the period when the switch is turned off. Where the switching device comprises a bipolar junction transistor (BJT) the collector voltage can fall below its emitter voltage, for example zero volts, causing the collector-base junction of the transistor to become forward biased, thus also taking the base voltage below zero. This can cause failure of the transistor, in particular the base-emitter junction of the transistor, and damage to a controller connected to the base, in particular latch-up.